A liquid Crystal Display (LCD) is known as the most popular display among all kinds of panel displays, in which a liquid crystal molecules of the LCD as an imaging element is non-irradiant. That is to say that the liquid crystal molecules do not generate light itself, but are only used as a reactor in a polarization field. The liquid crystal molecules are controlled by applying voltages to modulate transmission of light. In other words, the liquid crystal molecules can be regarded as a light valve for modulating the light intensity.
In particular, the Twist Nematic (TN)-LCD 10 shown in FIG. 1A and FIG. 1B comprises a top substrate 101, a bottom substrate 111, a top alignment film 103, a bottom alignment film 113, a top polarizer 105, a bottom polarizer 115 and a liquid crystal layer 107. Wherein, the liquid crystal layer 107 comprises a plurality of liquid crystal molecules. Two optical axes of the top polarizer 105 and the bottom polarizer 115 are perpendicular to each other.
Please refer to FIG. 1A. When a voltage is applied, the liquid crystal molecules of the liquid crystal layer 107 would be aligned in a vertical direction as shown. In this situation, the state of polarized light from the bottom polarizer 115 would not change when it passes through the liquid crystal layer 107. Hence, the light would be blocked by the top polarizer 105 and the transmittance would be zero.
Please refer to FIG. 1B. When no voltage is applied, the liquid crystal molecules of the liquid crystal layer 107 would be affected by the top alignment film 103 and the bottom alignment film 113. The liquid crystal molecules would be presented as twist arrangement as shown. Thus, when the polarized light passes through the liquid crystal layer 107, the state of polarized light would have 90 degrees change. Thereby, the polarized light can be transmitted through the top polarizer 105.
It is noted that the top alignment film 103 and the bottom alignment film 113 play decisive roles in the LCD 10. Between the interface of alignment film 103 (or 113) and the liquid crystal layer 107, many sorts of forces, other than the electromagnetic force, such as the van der Waals force, the dipole-dipole interaction, and the hydrogen bonding are involved to affect the alignment of the liquid crystal molecules.
Besides, some microstructures also have some abilities to align liquid crystal molecules. This phenomenon is described by Berreman theory. In present, the most popular alignment method is to rub (buff) a polyimide (PI) surface of the substrate. For example, in a Rubbing process, a PI (polyimide) material is firstly applied to the substrate 101 (or 111), and then a rubbing step is executed to form the alignment film 103 (or 113).
While skills for making the LCD 10 are developed continuously, how to increase the viewing angle of LCD 10 becomes an important topic.
When an observer looks at the LCD 10 at an oblique angle, light transmitted from the LCD 10 would arrive the observer's eyes at a tilt incident angle. In this case, because the light path is different from the default setting (a perpendicular incident angle which is shown as FIG. 1B), the light would present elliptical polarization or other polarization state, and then thereby the top polarizer 105 would have difficulty in completely absorbing the light that should be absorbed under the default setting. Hence, some light leakages occur and lead to the color contrast degrades of the prior LCD 10.
To improve the phenomenon described above, many solutions are provided. “Optical Phase Compensation” is one of the methods, which is economically and easily put into practice. The general concept of this method is to use a retardation plate for retarding the phase of the incident light so as to form a new phase that could be absorbed by the top polarizer 105. Consequently, the viewing angle of the LCD 10 under this effort is enlarged.
Retardation plates are generally divided into three types: 1) an A-plate, which has an optical axis parallel to its substrate; 2) a C-plate, which has an optical axis perpendicular to its substrate; and 3) an O-plate, which has an optical axis tilt to its substrate at a predetermined angle. A typical A-plate is described in U.S. Pat. No. 5,995,184. A typical C-plate is described in U.S. Pat. No. 5,528,400. A typical O-plate is described in U.S. Pat. No. 5,686,734.
Methods of making the A-plate are generally divided into two categories. One category of making the A-plate generally comprises two steps: 1) producing a film with polymer particles by extrusion or by solvent casting; and 2) applying tension to extend the film. For example, in U.S. Pat. No. 5,236,635, Toru Yoshida et al disclosed a method of making an A-plate by a polycarbonate (PC) material from a solvent casting. In U.S. Pat. No. 5,611,985, Hitoshi Kobayashi and Takao Saito et al disclosed a polysufone (PS) material. Though materials used in this category are relatively cheap, yet the film-extending step is painfully required. Besides, in casting the polymer particles, high boiling point and high polar solvent is needed to be used. Generally, the method of above is hard to achieve and procedures are complicated.
Another category of making the A-plate is to utilize a polymerizable liquid crystal material. A birefringence (two refractive index in two different direction) property of the liquid crystal material is suitable for making retardation plates. For example, in U.S. Pat. No. 5,995,184, Chung Young J et al disclosed how to make an A-plate 20 with a polymerizable liquid crystal material, as shown in FIG. 2. The method generally comprises steps of: 1) coating an alignment layer 203 to the substrate 201; 2) mixing the surfactant with polymerizable liquid crystal material; 3) applying a thin film of a mixed solution 206 to the alignment layer 203, and aligning the liquid crystal molecules 215 within the alignment layer 203 as shown in FIG. 2; and 4) polymerizing the thin film 205 to preserve the orientation by adjusting the temperature. Therefore, the surfactant could reduce the intrinsic tilt orientation of long axes of liquid crystal molecules 215 near the liquid crystal 205/air 209 interface, and the A-plate could be obtained.
Nevertheless, disadvantages of obtaining the aforesaid A-plate 20 of FIG. 2 are: 1) that a high temperature is required to form the alignment layer 203 onto the substrate 201, in which the substrate 201 definitely needs to be made of a heat-resistant material; and 2) that a rubbing step is usually applied to the alignment layer, from which some defects to the A-plate 20 are usually generated.
As described above, while the retardation plates become important in the LCD for obtaining an enlarged-viewing angle, the technique how to develop a device capable of optical retarding without sacrificing functions of the LCD is now an important issue in the art.